It has long been known that petroleum gases and liquids can be decomposed by the addition of heat. Such thermal decomposition has enjoyed extensive commercial application in the production of lampblack and certain petrochemical feedstocks. At one time, thermal "cracking", as it is called, was used to produce "town gas", an illuminating gas for domestic heating and lighting. Such thermal cracking processes produced a mixture high in illuminants such as methane and ethylene, and light residual oils, tars and cokes which were usually bothersome by-products. Steam was often added to such reactors, but with limited effect upon these problem by-products. In such thermal cracking of gas oil in the presence of steam, the steam and oil were usually passed over a hot structure, forming oil gas and leaving coke and tars in the reactor. The reactor was then blown with air to oxidize the carbon and heat the reactor bed to a temperature suitable for further introduction of steam and hydrocarbon. The process continued to operate in this alternating, push-pull manner, producing a high Btu product gas. Such generators were popular for many years, and still find application, but suffer the severe disadvantages of low thermal efficiency and poor control of gas quality inherent in discontinuous operation. Further, such discontinuous operation does not generally produce gas of sufficient quality to serve as input to sophisticated petrochemical processes, or synthetic methane plants. Examples of processes involving such discontinuity are found in U.S. Pat. No. 1,798,372; 3,365,387; and 3,641,190.
As the uses of refined products changed, and specifications became more sophisticated, it was necessary to develop new methods of treating petroleum liquids. Among the methods evolved was the gasification of oils to yield mixtures of light paraffins, olefins and elemental gases for fuel and petrochemical consumption. The invention disclosed here involves an improved process for the production of such gases from petroleum liquids including crude oil or oil obtained at the well head, gas oil, residual oil and the like.
Previous technology has produced two distinct approaches to the gasification of petroleum oils. In the most important of these processes, catalytic steam reformation, steam is reacted with vapors of normally liquid hydrocarbon over a suitable catalyst. The catalyst is employed for two distinct purposes: (1) to lower the temperature at which the desired reactions will occur at a reasonable rate; and (2) to encourage the preferential formation of certain of various possible products. However, a third very important effect of the catalyst involves the catalysis mechanism itself. In straight thermal cracking, reaction occurs by a mechanism involving free radicals. Such reactions can lead to polymerization and the precipitation of the coke. Catalysts initiate ionic reactions at acid sites, and free radicals do not naturally occur in the presence of such catalysts so long as relatively light feedstocks are employed. Consequently, by using the proper catalyst, one can minimize the problem of coke formation by the free radical mechanism. In steam reformation processes intended to gasify light liquid hydrocarbons (e.g. to produce syngas for petrochemical or fuels use), catalysts are used for the purposes just mentioned. By means of the catalyst, these reactions can be carried out in the usual range of 650.degree.-815.degree. C, and occasionally as high as 925.degree. C reaction temperature. A catalyst, often nickel-based, is chosen which has high selectivity for the formation of light paraffins, such as methane, and light olefins such as ethylene. Coke precursors are inherently lowered as a result of this selectivity and the ionic reaction mechanism noted above. By means of this catalytic approach, steam gasification of feedstock as heavy as naphtha (final boiling point near 250.degree. C) has been accomplished.
However, certain limitations of these catalytic steam reformation processes have been discovered. More particularly, most catalyst compositions require that the feedstock be relatively free of unsaturated hydrocarbons, which tend to deposit coke on the catalyst structure, causing physical degradation and reducing catalyst activity. This feedstock specification is met fairly readily with naphtha or lighter feedstocks, but as the molecular weight of the feedstock increases, the occurrence of olefins and aromatics rise dramatically, and the problem of coke precipitation is increased. Moreover, contaminants in the oils, such as sulfur compounds and metallic compounds, interfere with the reactivity of the catalyst. Both types of interference, by oil contaminants and by coke precipitation, are irreversible if the phenomenon is not noted and corrected almost immediately. Such degradation of the catalyst results in reactor malperformance, requiring shutdown and replacement of the often expensive catalyst. For this reason, catalytic steam reforming has been utilized commercially to gasify only substantially sulfur-free (usually less than 5 ppm weight) hydrocarbons of low end-point, even where this has required extensive pretreatment for removal of sulfur in the liquid phase.
The other and less important approach to the gasification of petroleum oils is based on non-catalytic oxidation of the hydrocarbons as a means of avoiding some of the aforementioned limitations of the catalytic processes. In these non-catalytic processes, air or purified oxygen is used as the oxidant, uniting directly with the hydrocarbons to produce oxides of carbon and releasing heat sufficient to cause thermal decomposition of the feedstock into desirable gaseous products. In certain of these processes, steam is also present and takes part in both oxidation and hydrogenation reactions. No means are provided to discourage the production of free carbon, and the affinity of the oxygen for carbon is assumed to be great enough to prevent excessive coke laydown. These processes are performed generally from 1390.degree.-1480.degree. C. This high temperature results in lowered thermal efficiency for the reaction as a whole, as there is no use for the waste heat at those temperatures. Also, most conventional heat exchange techniques do not work well at those temperatures, primarily due to materials considerations. Carbon residuals are usually maintained at about 2% of total carbon, and removed from the system mechanically, often on a continuous basis. Sometimes the carbon is taken up in a light oil slurry and recycled with the feedstock. Although reactor operability is not affected, thermal economy is further reduced.
The primary problem, however, is providing low-cost oxygen to these systems. If air is chosen as the oxidant, excessive dilution by nitrogen severaly limits the value of the product gas, and, in those plants utilizing pure oxygen, oxygen utility costs are often the highest operating costs involved. For reasons such as these, partial oxidation has not provided a suitable method for total, efficient gasification of high endpoint feedstocks not amenable to catalytic steam reformation.